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Objective. To investigate the flexion relaxation phenomenon in the thoraco-lumbopelvic muscles among a pain-free population when moving from an upright to a slump sittingposture.

Summary of Background Data. The presence of the flexion relaxation phenomenon (FRP) of the back muscles is well documented at end-range spinal flexion when standing. This phenomenon is commonly found disrupted in low back subjects. However, whether FRP occurs in sitting remains controversial.

Methods. The sample consisted of 24 healthy pain-free adults. Surface electromyography was used to measure activity in the superficial lumbar multifidus (SLM), the thoracic erector spinae (TES), and the transverse fibers of the internal oblique (IO) muscles while subjects moved from an erect to a slump sittingposture. An electromagnetic motion-tracking device simultaneously measured thoracolumbar kinematics during this task.

Results. There was a significant decrease in both the SLM and the IO activity when moving from an erect to a slump sittingposture (P = 0.001 and P = 0.004, respectively), indicating the presence of FRP. TES activity was highly variable. While 13 subjects exhibited an increase in activity (P = 0.001), 11 demonstrated a decrease in activity (P = 0.001), indicating the presence of FRP. FRP occurred in the mid-range of spinal flexion for the SLM, IO and TES when present.

Conclusion. The findings show that the SLM and the IO are facilitated in neutral lordotic sitting postures and exhibit FRP at mid range flexion while moving from upright sitting to slump sitting. These findings show that FRP in sitting differs from that in standing. Variable motor patterns (activation or FRP) of the TES were observed. These findings suggest that sustaining mid to end-range flexed sitting spinal postures result in relaxation of the spinal stabilizing muscles.

Flexion relaxation phenomenon (FRP) was investigated in trunk muscles among healthy subjects while moving from an upright to a slump sittingposture. FRP consistently occurred in the superficial lumbar multifidus and the internal oblique muscles; however, there were variable motor patterns in the thoracic erector spinae muscles. When FRP was present, it largely occurred at mid range spinal flexion.

From the *School of Physiotherapy, Curtin University of Technology, Perth, Western Australia; and †School of Rehabilitation Sciences and Physiotherapy, Ghent University, Ghent, Belgium.